1. Field of the Invention
This invention relates to the exposure of resists in the formation of integrated circuits, and more particularly to masked ion beam lithography exposure systems and methods.
2. Description of Related Art
Integrated circuits are generally formed with a series of masks that are used to create desired patterns on a semiconductive substrate. The substrate is initially coated with a resist material, and a mask carrying a desired pattern is positioned over the coated substrate. The substrate is then exposed, fixing the resist except where it has been shaded by the mask pattern. The shaded resist is then washed away in a solvent bath, and any underlying layer etched to reveal the substrate (for negative resists, the unexposed portions are washed away). Appropriate dopants or materials are next applied to the substrate, followed by the removal of the remaining resist, the application of another resist layer, and another mask iteration. This process continues for successive masks until the circuit pattern has been completed.
Ultraviolet light is commonly used in conjunction with a photoresist in fabricating integrated circuits. More recently, however, ion beam lithography techniques have been developed in which H.sup.+ ions are used to expose an ion-sensitive resist material on the semiconductive substrate. Ion exposures yield much higher resolution patterns than those produced by ultraviolet light because, at submicrometer dimensions, light undergoes substantial diffraction whereas ions are not substantially influenced. Much of the basic masked ion beam lithography (MIBL) technology is disclosed in U.S. Pat. No. 4,101,782, R. L. Seliger, "Process For Making Patterns In Resist And For Making Ion Absorption Masks Useful Therewith", issued July 18, 1978, and U.s. Pat. No. 4,158,141, R. L. Seliger et al., "Process For Channeling Ion Beams", issued June 12, 1979.
Ion implantation accelerators have previously been used for doping semiconductors in the fabricating of integrated circuits. These same machines have also been adapted for experimental use to provide the necesssary ion exposure beam for an experimental MIBL system. While convenient because the equipment is already available, ion implantation accelerators are very large and overpowered for ion beam lithographic applications. Serious problems have been encountered in aligning the beam with the mask and the underlying substrate, in providing appropriate beam power, and in the expense of an exposure.
With the use of new channeling masks such as that disclosed in U.S. Pat. No. 4,158,141 to Seliger et al. and assigned to the assignee of the present invention, the mask must be held in accurate angular alignment with the beam. If the beam angle changes, it has been necessary in the past to tilt the mask by a corresponding amount. This in turn has required a similar tilting of the wafer and the entire wafer stage, a cumbersome procedure.
To generate the correct beam size, shape and collimation for illuminating a mask, a series of apertures through which the beam is passed during transit from its source to the mask have generally been employed. The size and shape of the apertures are selected so that they transmit only that portion of the beam which will produce the desired beam collimation. The rest of the beam around each aperture is blocked and wasted. This results in a loss of efficiency, since much of the beam whch is originally generated never reaches the mask or substrate. With system voltage differentials of up to 300 kV, the power loss can be significant.
With past systems it has also been difficult to obtain a pure H.sup.+ beam. Undesired particles in the beam can bombard and heat the mask, resulting in distortion and misalignment, and can also be lead to undesired resist exposure. Such unwanted particles include H atoms and molecules, metal impurity ions resulting from sputtering inside the ion source, and sometimes ions from impurity gases.
In addition to MIBL, various alternate lithographic techniques have been explored. These approaches include direct-write electron beam lithography (EBL), x-ray lithography (XRL), and focused ion beam microfabrication. Each of these techniques has limitations that make a more efficient and practical MIBL system desirable. EBL is not well suited for high speed printing in the submicrometer feature size domain because of its slow, serial patterning with a finely focused beam. EBL systems used for integrated circuit fabrication are characterized by high cost and complexity, low throughput and marginal resolution. The emergence of XRL for integrated circuit fabrication has been delayed because of low throughput for submicrometer features. High resolutions can be obtained, but XRL is made difficult by the low x-ray absorption of high resolution resists, and by the low x-ray flux available for high resolution exposures. The possibility of using synchrotron emissionn as a source for x-ray exposures has also been considered, but involves problems of very high capital investment, operational costs, complexity, reliability, and radiation safety.